Electrical cables used to transfer audio, data and radio frequency signals are characterized by capacitive, inductive and resistive properties. Collectively, these properties establish the impedance of the electrical cable. One or more of these three properties can cause degradation of the original signal as it is transferred through the cable. It remains a great challenge in the electrical cable industry to minimize and/or carefully control each of these properties over a broad range of frequencies.
A well known technique to limit the inductive properties of an electrical cable involves the use of twisted pair construction. A conventional twisted pair cable design incorporates a pair of insulated conductors twisted around each other in a helical fashion. One advantage of twisted pair cable construction its reduced sensitivity to common mode magnetic interference. This reduced sensitivity occurs when one conductor is designated to carry a signal source current and the other conductor is designated to carry the signal return current. As the opposing currents travel through their respective twists, the opposing magnetic fields cancel each other at the interior of the cable. This cancellation, in effect, reduces the total loop area available within the circuit created by the electrical cable. With a smaller effective loop area achieved by adding additional twists, not only is the amount of magnetic energy stored and radiated reduced when compared to non-twisted electrical cable, but a smaller effective loop area also means the electrical cable is less sensitive to interfering magnetic fields. It is in this fashion that the effective inductance of a twisted pair cable is lower than the inductance of a non-twisted cable having the same overall length.
While twisted pair cabling has the inherent advantage of reduced magnetic fields, there are still improvements necessary to maintain complete minimization and/or control of the reactive portions of the cable impedance. Capacitance in a twisted pair design will depend on conductor length, cross sectional area, distance between conductors and the dielectric constant of the material used to insulate the conductors. As twists are added to a cable of fixed length, more conductor and insulating material will be required and capacitive coupling between conductors will increase as a result. So, while the total magnetic field will be reduced in a twisted pair design, a natural consequence is an increased amount of electric field energy stored in the dielectric medium, a phenomenon known as capacitive coupling, within the proximity of the two conductors.
For any required value of capacitance, an insulating material with a low dielectric constant separating source and return conductors allows these conductors to be arranged in closer spatial proximity to each other than the same conductors separated by a material with higher dielectric constant. Therefore, while the two dielectric materials can produce two different cables having the same measured value of capacitive coupling, the design utilizing the low dielectric constant insulator can result in a cable that has a lower measured inductance.
Various means of reducing capacitive coupling between the conductors of twisted pair cables are known to the art. Air is considered to be the best dielectric element for the purposes of reducing capacitive coupling. Air is also one of the most difficult elements to incorporate in an electrical cable design. Several techniques for incorporating air as a dielectric element have been disclosed. U.S. Pat. No. 1,305,247 discloses a strip of flexible insulating material separating conductors and maintained in an elastic and compressible condition. The insulating material can be formed with a continuous central hollow or air cavity. U.S. Pat. No. 2,804,494 discloses a twin lead twisted pair RF cable utilizing an elongated hollow tube with grooves formed in diametrically opposite sides of the tube and extending the length of the cable. Conductors are positioned in open spaces formed by the groove and rely on spacers to maintain the conductors in a constant spatial relationship.
When designing a cable to be used for high fidelity signaling an additional concern is the phenomenon of eddy currents known to cause a conductor to increase its resistance in proportion to the frequency being transmitted through the cable. Higher frequency signals will result in a current density that is concentrated at the surface of a conductor where these is less conductor area available. This effect is known in the art as skin effect. Skin effect contributes an additional measurable component of AC resistance to the over all impedance properties of the cable.
Litz wire techniques are well known in the electrical industry for reducing power losses due to eddy currents and the resulting skin effect. Power conversion circuits that utilize transformer or inductor windings make use of Litz wire properties to improve power efficiencies at high frequencies, typically less than 1 MHz. Among other techniques exploiting skin effect, U.S. Pat. No. 4,538,023 describes using conductors of various diameters with smaller diameter conductors surrounding larger diameter conductors as a means for adjusting the relative speed of high and low frequency components.
The twisted pair configurations which incorporate a continuous air channel suffer from several major limitations. In the previously mentioned patents, a continuous air cavity of significant cross sectional area with respect to the conductor diameter must be used to prevent deformation or collapse of the air cavity under the compression forces achieved during the twisting process and the compression forces exerted during normal bending necessary to conform the cable to environmental and installation conditions. An air gap that is larger than the minimum necessary to achieve a desired impedance results in a larger loop area with a consequentially larger inductance. In addition, these techniques require relatively complex and expensive manufacturing methods. Other known methods of providing substantially solid insulating strips between conductors do not incorporate any method for providing air as a substantial dielectric element. Moreover, these strip and air cavity dielectric techniques do not incorporate a specific means for reducing skin effect. Previously mentioned Litz wire designs intended to minimize skin effect do not address a specific means for incorporating air to reduce the capacitive effects between the signal source and return conductors of an electrical cable.
Accordingly it is desirable to have an electrical cable that overcomes these limitations and concerns.